1. Field of the Invention
The invention relates generally to the field of wellbore drilling. More specifically, the invention relates to methods for selecting the type of drill bit used, and the drilling parameters used to drill a wellbore so as to optimize the overall drilling performance.
2. Description of the Related Art
Methods known in the art for selecting the type of drilling bit are typically based on analysis of data related to the drilling performance achieved on previously drilled wells in the vicinity of the wellbore being drilled (called xe2x80x9coffset wellsxe2x80x9d), and are based on monitoring and analysis of dull (xe2x80x9cworn outxe2x80x9d) drill bits. Other methods known in the art for bit selection include methods for simulating (xe2x80x9cmodelingxe2x80x9d) the formation xe2x80x9cdrillabilityxe2x80x9d and other drilling performance parameters. The drillability of earth formations is related to mechanical properties of the formations, in particular the compressive strength. Knowledge of the mechanical properties is useful to optimize the drilling of the formations. One well known method to determine compressive strength is based on acoustic (xe2x80x9csonicxe2x80x9d) well log interpretation combined with lithological analysis of formation data. Even though it is sufficiently reliable to calculate rock strength, the prior art method has two limitations: first, that the compressive strength is derived in this method from elastic theory, relating the rock acoustic responses to rock hardness, therefore the measurement is not a direct determination of strength; and second, that the acoustic log is recorded after the formation is completely drilled, and is therefore not useful for predictive analysis in the particular wellbore being drilled.
The invention is a method for selecting at least one drilling parameter for drilling a wellbore through earth formations. The method includes determining a loading displacement relationship of samples of earth formations from measurements of loading displacement made on the samples by an indenter. The at least one drilling parameter is selected from the loading displacement relationship thus determined.
In one embodiment of the invention, the loading displacement relationship is determined from drill cuttings made during the drilling of a wellbore. The relationship thus determined during drilling is used to select the at least one drilling parameter during drilling of the wellbore to improve drilling performance.
Drilling parameters which can be selected by the method of the invention include, but are not limited to, mill tooth and/or insert bit type when roller cone drill bits are used; whether and what type of gauge protection is to be used on the drill bit; type, size and orientation of jet nozzles to be included on the drill bit; and where fixed cutter bits are used, the blade structure, cutter type and density as well as the cutter impact resistance can be selected. Other drilling parameters which can be selected using the method of the invention include weight on bit, drill bit rotation rate, and drilling fluid flow rate.
In a particular embodiment of the invention, values of a slope of displacement with respect to applied force for the loading displacement measurements on each of the samples are characterized with respect to a measured compressive strength for each of the samples. The characterization is used in this embodiment to determine a compressive strength for samples of rock during drilling of the wellbore.